It is secreted into the blood and degrades large vWf multimers, decreasing their activity, hence ADAMTS13 acts to reduce thrombus formation.

[5] Since 1982 it had been known that thrombotic thrombocytopenic purpura (TTP), one of the microangiopathic hemolytic anemias (see below), was characterized in its familial form by the presence in plasma of unusually large von Willebrand factor multimers (ULVWF).

[5] In 1994, vWF was shown to be cleaved between a tyrosine at position 1605 and a methionine at 1606 by a plasma metalloprotease enzyme when it was exposed to high levels of shear stress.

In the next two years, the same two groups showed that the congenital deficiency of a vWF-cleaving protease was associated with formation of platelet microthrombi in the small blood vessels.

[5] Deficiency of ADAMTS13 was originally discovered in Upshaw Schulman Syndrome, the recurring familial form of thrombotic thrombocytopenic purpura.